I want to share something I found by tinkering with car audio subwoofers. I see no reason it wouldn’t apply to home audio as well, and maybe this is already known but it’s new to me and I wanted to share:
A couple years ago I got a 15” sub in a prefab ported box for my car. It has a slot port with a 90 degree bend in it with the port terminating next to the sub on the front surface of the box. Tuning is probably mid to high 30s. This is my best guess but I don’t know for sure as I haven’t taken the time to model the box. I went back and forth between ported and sealed by closing off the port with wood. I wasn’t quite happy with either configuration. Sealed had great transients but it needed gobs of power and it seemed kind of anemic with some of the low notes. On the flip side, the ported configuration didn’t require as much power and hit the low notes with authority. The downsides were that the transients were a bit rounded off and it was a bit boomy with excess overtones / harmonics / high frequency “ringing” or whatever you want to call it. It sounded kind of nasally.
I attempted to tame the ported box by putting some polyfill in the box and then put some insulation in the port (this is the secret sauce that inspired me to make this post). The insulation I used was a sheet of insulation approx. 1-2” thick from a Hello Fresh order to be precise. I only put it in half the port which was in the rear of the box up to the 90 degree bend and leaving the port exit and the length after the 90 degree bend empty. The width of the port is about 3-4 inches so the insulation takes up about half the cross sectional area.
The sound, in my opinion, is beyond amazing. It’s tight and controlled while going deep and loud. It’s like I’m getting the benefits of sealed with the output and low frequency extension of ported with hardly any of the “nasally” ported sound. I much prefer it to either sealed or ported (without port insulation). The blend with the main speakers is also excellent.
Here’s what I think is happening from my limited understanding of the math behind ports: First, the air inside a port resonates as the subwoofer moves and because it’s just a slug of air in the port there isn’t much to dampen it until we add some insulation. I think the insulation acts as a dampener on the air mass which gives it such good transients and reduces the nasally, ringing sound that ported subs can sometimes have by making everything settle down quickly after the bass note stops. Second, I think the insulation lowers the tuning frequency of the box by reducing the cross sectional area. Normally this could be a problem because it can introduce chuffing noise, but since the insulation is only in the back of the box/port, the velocity at the port exit is still normal and I don’t notice any chuffing noise.
I’m only sharing this now because I tried a different subwoofer (supposedly much better) and ran into the same issues with sealed and ported sound until I did the insulation trick which made me realize just how important that step is. I’ll never run a sub without some type of port insulation strategy again.
Has anyone else had similar experiences or have anything to add?
